1. Field of the Invention
This invention relates to a control circuit for use in an inductive circuit wherein a semiconductor switching element is repetitively and rapidly switched between its conductive and non-conductive states to hold the voltage across the switching element to substantially twice the input voltage and in addition to recover and return to the input source the energy stored in the inductance and wiring of the inductive circuit.
2. Description of the Prior Art
Many applications exist for the use of rapid on/off switching of unidirectional current in an inductive circuit. Rapid alternate interruption and resumption of current causes problems of excessive power loss in the switching element, and the appearance of potentially damaging voltage transients across the switching element.
One such rapidly switched inductive circuit is a DC-DC converter such as the prior art converter shown in FIG. 1. The converter receives a DC input from a source and transmits substantially DC power to a load. DC to DC converters are useful for transforming the voltage level between the source and the load, and for providing electrical isolation of the load. This transforming and isolation capability is useful in providing DC power to DC logic circuitry, which requires a relatively low and stable DC input voltage.
Referring to FIG. 1, a DC to DC converter includes a primary circuit 50, a transformer 12 and a secondary or load circuit 52. The primary circuit 50 is coupled to a DC source 10 which provides power to the primary winding 18 of the transformer 12. A transistor switching element 20 is in series with the primary winding 18. The base of the transistor is coupled to a source 16 of a switching waveform which operates to drive the transistor rapidly between its conductive and nonconductive states. Typically the switching waveform is such as to drive the transistor 20 between its conductive and nonconductive states at a frequency of approximately 20 kilohertz.
The rapid on/off switching of the primary circuit 50 causes power having a net DC component to flow by induction through the transformer 12 to the secondary or load circuit 52. The power input to the secondary circuit flows through a rectifying diode 24 and a relatively large filtering inductor 26 to the DC load represented as 30. Capacitor 34 which is in shunt relationship to load 30 forms with conductor 26 a filter to provide the DC current to load 30. Another diode 28, sometimes referred to as a "free wheeling diode", is provided in parallel with the load to afford a return path for current induced by the filtering inductor 26 during periods of the switching cycle in which the primary current is interrupted.
Free wheeling diode 28 maintains current flow in inductor 26 during those periods of the switching cycle when transistor 20 is nonconductive. This maintenance of current flow in inductor 26 prevents the discharging and subsequent recharging of the inductor during the cycle. Diodes 24 and 28 cannot conduct simultaneously as such simultaneous conduction may result in a short circuit. One method for preventing simultaneous conduction of the diodes is by loosely coupling the primary and secondary windings of transformer 12. This loose coupling of the transformer windings controls the rate of change of current through free wheeling 28 so that its stored charge may be recovered before forward diode 24 conducts.
Energy is stored in the transformer leakage and shunt inductance and wiring of the DC to DC converter circuit during conduction of the transistor switch 20. Some of the energy stored in the shunt inductance is used to reverse the voltage across the winding to thereby reset the flux in transformer 12 when switching element 20 is nonconducting. Resetting of the transformer flux begins when transistor 20 switches from its conducting to nonconducting state.
Upon switching of the transistor from its conducting to nonconducting state potentially harmful voltage transients may occur across the transistor. Additionally in order to minimize the losses which occur upon the switching of transistor 20 from its conductive to nonconductive states and vice versa it is common to affect the making and breaking circuit switching operations with the maximum switching rates attainable. Fast opening and closure of the circuit reduces the time during which the switching element exhibits an intermediate impedance, i.e., between substantially infinity (open circuit) and substantially zero (closed circuit). Since most of the switching losses are caused by current flowing through the transistor while it exhibits this intermediate impedance, the minimization of the time during which this conduction obtains is considered essential. During the circuit opening, or "breaking" portion of the cycle the extremely fast switching operation aggravates switching loss and causes over voltage problems. With extremely fast switching a high rate of current decrease with respect to time causes the magnetic energy stored in the inductance of the transformer 12 to induce a large voltage transient across transistor 20. During this time, practically the entire voltage across the primary of the transformer 12 is applied to transistor 20, and this voltage is applied when the transistor is in its vulnerable intermediate impedance state and immediately succeeding that state.
Several approaches have been used in the past to limit the transient voltages and also minimize switching losses. One such approach is to reduce the inductance of the transformer primary and thereby reduce its troublesome stored magnetic energy. Reduction of the transformer primary inductance may avoid excessive voltage transients induced by the transformer primary leakage inductance during the turn off protion of the switching operation but a high transformer series inductance is desirable during turn on and in addition for preventing simultaneous conduction of the diodes 24 and 28.
Another approach involves the use of "snubbers" connected to the switched circuitry. One such example of a snubber circuit is shown in FIG. 1 wherein a series circuit comprising a capacitor 13 and a resistor 14 is connected in shunt relationship with transistor 20. This snubber circuit 32 functions in a manner such that on turn off of transistor 20 the voltage across the transistor is prevented from rising to a value which may be harmful to the transistor. During the period of time when transistor 20 is conducting the voltage across snubber capacitor 13 is essentially zero as the voltage across the transistor is essentially zero. When transistor 20 is turned off by the waveform generated from source 16 a current which results from the energy stored in the transformer inductance flows through snubber capacitor 13. Snubber resistor 14 is sized such that the voltage developed by the current does not exceed the voltage ratings of transistor 20. The time constant of snubber circuit 32 is related to the size of the leakage inductance of the transformer. When the leakage inductance of the transformer is large a relatively large snubber capacitor is needed.
One drawback associated with snubber circuit 32 is that while this snubber circuit is desirable for limiting transients during turn off of transistor 20 it is not desirable during turn on of the transistor. During turn on the snubber capacitor 13 is charged. As the voltage across transistor 20 when it conducts is essentially zero, the energy stored in the snubber capacitor 13 must be dissipated in snubber resistor 14. Even during turn off the snubber capacitor is charged through snubber resistor 14 resulting in losses occurring in that resistor. It is therefore seen that while snubber circuit 32 may result during turn off in a decrease of transient voltages and an improvement of circuit efficiency resulting from a reduction of switching losses the energy dissipated in snubber resistor 14 during turn on and turn off may well counteract the improved efficiency.
Another problem associated with the use of snubbers is exhibited where a relatively light load is driven by the converter. Under light loads waveform alternations caused by the use of snubbers can interfere with the stability and precise control of the output voltage and power. This is because, at light loads, the snubber induced waveform changes become significant with respect to the waveform of power actually transferred by the converter to the load.
Snubbers also contribute to undesirable "device sensitivity" of their associated circuitry. Device sensitivity is that sensitivity which performance of a circuit exhibits with respect to small variations in the properties or characteristics among replacement semiconductor and other circuit elements which are nominally identical. The effectiveness of the snubbers may be affected by the mere exchange of one circuit element for another nominally identical element as minor variations in circuit elements influence transient waveforms. Snubbers depend for their effectiveness upon the modification of such transient waveforms.
The use of any snubber circuitry entails a compromised design between improving circuit performance on the making of the circuit, and on the breaking operation. To improve circuit making performance entails some sacrifice of breaking performance, and vice versa.
Another circuit for holding the voltage across the transistor switch of a DC to DC converter to a predetermined magnitude during turn off of the switch is described in a paper by J. A. Bassett entitled "A Biased Core DC-DC Converter" which has been published in the IEEE International Telecommmunications Energy Conference (INTELEC) Record 1979, pages 253-257. This circuit utilizes an energy recovery winding which provides extremely close coupling to the primary winding of a main transformer constructed from a toroid.
Toroidal construction has certain disadvantages as compared to main transformers which are constructed from regular coils and ferrite cores. In particular toroids are harder and more expensive to wind than regular cores and for dc-dc converters designed to produce large amounts of output power the toroids, due to the wire size involved, may have to be wound by hand. Further toroids as shown in FIG. 4 of the above paper are usually potted in order to present a more aesthetically pleasing appearance to the end user of the converter circuit. It is therefore desirable to construct a dc to dc converter circuit which limits the voltage across the transistor switch during turn off as well as recovers the energy stored in the circuit inductance and yet utilizes a transformer which can be simply and inexpensively constructed from regular coils and cores.
In accordance with the present invention there is provided circuitry which limits the voltage across the transistor switch on turn off to substantially twice the voltage of the d-c source and also recovers and returns to that source energy stored in the circuit inductance without the necessity of using a main transformer design which involves complexity and expense in construction.